High-lift airplane



Feb. 13, 1951 O c KOPPEN 2,541,704

HIGH-LIFT AIRPLANE Filed Sept. 2, 1949 Patented Feb. 13, 1951 UNITED STATES PATENT OFFICE HIGH-LIFT AIRPLANE Otto C. Koppen, Wellcsley, Mass., assignor to Hello Aircraft Corporation, Canton, Mass., a corporation of Delaware Application September 2, 1949, Serial No. 113,704

6 Claims. (Cl. 244-13) 1 This invention relates to fixed-wing aircraft for small-field use, and is particularly adapted to military liaison and rescue operations.

The utility of present-day fixed-wing aircraft for military as well as civil use is very much 5 of the coefficient depends on the proportion of limited by the fact that they must use relatively the Wing span covered by the high-lift devices large airports that are generally located at conand the particular type of high-lift device emsiderable distances from points between which ployed. (Lift coefficient is defined as, transportation is actually desired. Since 1133116 C I lift ideal of personal transportation is a door- 0- L= door vehicle, the closer the airplane approaches wmg arewg an densltyxspeed squared this ideal the more utility it will have in every- With wing boundary layer control, even higher day transportation. lift coefficients appear possible, but a considerable The chief reasons for the lack of accessible amount of additional development Work is relanding strips are the lack of availability and the quire before p ct cal appli a ion of th m ans cost of the size of landing strips required. Even of increasi e lift OOBfilCient can e m de. a very light present-day airplane requires a strip Despite the old inventions and use of high-lift of 1500 feet between 50 ft. obstructions and a devices, and prior aircraft Capable of flying e w smooth surface at least 1000 feet long by 50 feet a speed of 35 miles per hour even without the use wide. The purpose of this invention is to provide of a high-lift device, the practical art has not a practical single-propeller fixed-wing airplane p v ed a s s ctory fi edWing aircraft for that will be capable of using much shorter strips, small-f eld use because, though a low landing say a strip of but 400-600 feet between 50 ft. speed was secured, a low take-off speed was unobstructions. with a relatively smooth or prepared known. Even to achieve such landing p surface of 200 feet by 50 feet centrally of the a c W a plain W s. a Wing loading Of 1985 length of said trip. than 5 lbs. per square foot is required. 'The As a practical matter, in order to be able to use amount of wing span and area required for this such a surface but 200 feet long, the proposed airlow wing loading becomes prohibitive from weight plane in taking ofi" must be capable of acceleratand cost standpo nts and, moreover, the airplane ing to its take-off speed (and, in landing, dewould be impractical for a speed range greater celerating from its touch-down speed) in about than about two because of the high accelerations one-half of that distance. That this landing perthat would be experienced in gusty air. Moreformance is possible with the use of slots and flaps over, experience has shown that it is impossible was demonstrated long ago in the Guggenheimto handle lightly wing-loaded airplanes (e. g. Safe Airplane Competition of 1930 wherein both 5-7#/sq. ft.) on even moderately windy days. the Curtiss and the Handley-Page entries stopped Although the high-lift means of providing a in less than 100 feet after touch-down. However, steep glide path, low landing speed, and a short in still air, a touch-down speed of less than 35 landing roll, are all old in the art. these factors miles per hour is required in any case to allow alone are not sufficient to provide satisfactory the ordinary pilot to judge the landing accurately. performance for the small-field airplane. Un- At the present-day common landing speed of less the airplane can also take-off from the same miles per hour, a plus error in timing of only area and over the same obstructions the advan- 2.6 seconds may cause the pilot to overshoot entages of a steep descent path and low landing tirely the 200 ft. surface. During an approach speed are of no help in the utilization of small to a landing, a pilot must make continuous esti- 4 landing fields. In the case of the two leading mates and then adjustments of the sinking speed Guggenhein Competition contenders previously and, since the accuracy with which he makes admentioned, neither could do more than barely fly justments varies in proportion to his distance level with full power in the high-lift condition. from the landing spot, the more time that is The same was true of the McDonnell airplane available, i. e., the lower the speed, the more 50 as tested by the National Advisory Committee for accurate will be his approach. Therefore, both Aeronautics (N. A. C. A. Tech. Note No. 398). for the approach and landing run accuracy, a Consequently the high-lift devices could not be low speed is greatly desired, since these two used during take-off and therefore take-offperfactors in the aggregate fix the length of landing formance was little if any better than that of an strip required.

The means of obtaining low landing speed are 2 old in the art. With leading edge slots and slotted flaps, maximum lift coefficients from 2.5 to 3.2 (NACA Absolute Units) can be obtained and provide the low landing speed. The value unflapped airplane.

The winner of the 1934 International Touring tained at the very considerable expense of a power loading (weight *per horse-power) of 6.75 pounds per horse-power, about twice the power installed in a comparable civil airplane. Every horsepower installed in such an airplane weighs. abouttwo pounds and requires about 1.4 pounds of gasoline for a satisfactory endurance.-

pound of power plant and gasoline, each additional horse-power installed costs 4.7.6 pounds of;

gross weight increase. The ratio of pay load to gross weight then becomes proportionally-reduced making th airplane unduly large and expensive in relation to its payload; The operation "costs: are also increased substantially by the: additional:

fuel-cost, insurance, depreciation; and storage;

The fundamental -reason* for the requirement of.-.increasedthrust when flying at high-lif t too-i efiicients is ,that-thednduced drag increases as theesquar-e'of rtheJift'cQefELcient. Therefore; if: the lift coenicient -is. doubledtheiinduced drag coeificient is multiplied, by four. To keep the induced .drag coefficient constant-when the lift is doubledjthe spanwouldalso-have to'be doubled'for constant w-ingarea.:. This would-quad ruple the. wing aspect. :ratio, I

To-havethe same induceddragcoefiicient as a plain wing .of- 36 ft. span, the .spanv would have n to be increased.to..72 .feet-,.with.the.-tail length increased in. proportion. This-.would: obviously 40 required to overcome. the induced drag.=. In ad.-' dition, to increasingthe induceddrag, slots and flaps also increasethe profile. drag, which .no;

be animpracticaLmethod of. reducing thepower known. alteration .of fthe. .structure will; reduce. Moreover, it is undesirableto attempt to reduce the drag ,at high-lift coefficients .unless. a..sep..

arate lift and drag-control can .be provided since.v a "high drag at high-lift "coefficients. is helpful.

during-landing.

I have discovered that. I lcan. avoid the. .use of; 1. a large and heavy aircraft by,a particular com:- bination ,of engine, propeller,. ,and .wing .dimen-.. sions in an airplane with high-lift devices and. I'have found thatthe..high.,.drag ,(both inducede andwprofile). of my wing .withits effectivehighlift device canloe ,overcome-byproperly. relating the physical dimensions .of .-.the propeller with -re-..-

spectto. said wing... This Lhavedoneby. corr.e-.

lating propeller, dimensions. in.,relation,to those.

of the wing so that prop. dia. Xpropsarea wing span Xwing area andwith propeller diameteriingfeet related.to

engine power so that D2 Brake-horsepower? This hasprovedto provide the desired results. 7

because fundamentally, the propeller diameter is the diameter of the. column of air accelerated to producethe-thrust and-because. the span. of the wing,is..the diameter of ,the column of .airdefiected downwardly to produce lift audit will be seen that. the,.firs.t. formula. includes the ratio SinceQabOut-K 0.4 pound of structure is required tdcarrycach of the propeller diameter to the wing span. In addition, the amount of rearward acceleration of air by the propeller depends on the blade area, and the downward deflection of the lift column depends on the wing area. These factors are related in the first formula which gives no indication of the amount of-power required to produce the desired'result,*but this is controlled by the second formula which relates the required power to propeller diameter.

The. present invention furnishes a practical solution for the conflicting considerations above set forth and embodies a novel combination in asingleepropellrfixed-wing airplane which, with normal power loading, provides short takeoff and steepclimb, as well as satisfactory cruise, steep glide and slow landing characteristics, including short landing roll with the use of brakes. This practical solution gmoreover contemplates an .airplane.-.easily handled and controlled by :an ordinary pilot;

In thedrawings:

Fig.1 isa side-elevation. illustrating an airplaneof-this invention;

Fig.2 is a .topplan view .of the same, partly.-

broken-. away andv Fig... 3. is .a cross-sectional elevation illustrating ..certain.-.parts. of the same.

Referring to'the illustratedairplane, the drawings show amodification .of. a present-day widely used. civil. airpla'ne...(kno.wn .as a. Piper J.-3 Cub-- with a wing span of 35.3 ft. and.wing.. area of.

178.5..sq. ,ft.) .having a. conventional. body 1 a and empsnnag l2 jconsisting of vertical .fin, rudder, stabilizer. and .elevator, .as shown. The landing gearof. usual type includes main wheels l i supporting the, airplane through hinged legs is and.

adde'dshockstruts. -l8,.with tailwheel 253. The

wing .38 attached .to thebody it includes left and right panels (as shown).modified to provide a slot betweenv said wing and air-.foil-shaped slat 3|".mounted thereon. The entire trailing edge, of said wing isv equipped with fiaps 32 mountedon the wing having suitable. manually-actuated operating controls for each (not shown) so that the flaps may be depressed and held in the highlift position indicated by ,dashedlines (Fig. 1)..

so as to provide a maximum lift .coeflicient preferablyiof, at least 2.5. power-off, and as high as possible... Spoilers 34 having suitable manually.- actuated operating... controls (not shown) are preferredas rolling control, though ailerons may be used either alone orin conjunction with spoile ers., Referring to.Fig. 3,. the spoilers 34 are are. 'rangedfor differential travel below as .wellas above .theupperasurface .of .the'wing resulting in a more naturalffeel. to. the controls, by proe vidingarecess 35 into whichthelowered spoiler may travel while the spoiler. 34 on-the opposite wingis-in araised position (as shown by dotted lines), in which it is effective as a rolling control.- The slat 3 I may be either fixed (as shown) or.-.retractible, as well known in the art, and mounted on or in the wing at the leading edge thereof, all of thesevarious forms, for simplicity, beingherein referred to as mounted on said wing. .The particular airplane illustrated has a wing loading ;of7-8#/sq. ft" but the advantages of the invention. may behad in airplanes 'of heavier wing loadings.

The preferredairplane shown'is powered by apsingle typical flat multi-cylinder,engine 4t:

driving a -multi-groove- V.-belt pulley 42 which, through V-belts 44, drive a much larger V-;belt pulley; 46:.(rotatably mounted on. a counter shaft not shown) and attached to the single variable pitch propeller 43, thus gearing down the propeller as compared with the engine. Conventional reduction gearing may, of course, be em ployed in lieu of the V-belt reduction shown and described, and it is contemplated that either one or two engines may b geared to the single propeller.

It will be observed that the propeller as shown is very much larger than is conventional (the reasons for which will presently appear). Also, for satisfactory cruising speed, said propeller should be of variable pitch (provided by usual operating mechanism, not shown) so as to afford steep pitch for cruising conditions and flat pitch for take-off and climbing conditions. The propeller is given sufiicient ground clearance in the three-point ground attitude of the airplane (in the particular airplane as shown in Fig. 1 with 13 angle of attack) by its elevated thrust line in which attitude it can be both taken off and landed. Thus, the ground clearance problem is not acute since this airplane of this invention can operate from the three-point attitude of a conventional landing gear (Fig. 1). It is a vital feature of the invention that there is provided sufficient thrust so that the airplane will be able to take off in still air at a distance of the same general order of length (e. g. 100 feet or thereabouts) as that of its braked landing roll, and also climb satisfactorily at an. angle of attack at least at great as its threepoint ground attitude. The preferred airplane of this invention has sufficient margin of thrust over drag to produce a 0.25 gravity acceleration at take-off air speed which will provide an anle of climb at said airspeed of approximately 15 in still air. The surprising results obtained are virtually independent of the slipstream effect as a practical matter, for example, the principles of my invention are equally applicable to a single-propeller pusher aircraft.

Heretofore, efforts to provide short take-off have involved abnormal power-loading (highhorsepower), much increased size of airplane, or both, as the result of failure to appreciate the importance and necessity of high thrust at low speed secured through large propeller disk area per horsepower as employed in accordance with this invention.

Propeller thrust per horsepower at any speed,

V, (in accordance with NACA Wartime Report No. 3G26) can be expressed as:

where T is thrust,

BHP is brake horsepower,

Kt is modified thrust coefiicent N is the propeller R. P. M., and

D is the propeller diameter in feet.

At first it might appear that this expression is contradictory to the conception that a high thrust per horsepower requires a large diameter, i. e., the diameter appears in the denominator and therefore if the ratio of Kt/N were constant, the highest value of would be obtained with a minimum diameter. However, the ratio of Kt/N is also dependent on the diameter and will have its maximum value when Kt is a maximum and N is a minimum.

Also, the relation between Kt and the power coefiicient is such that the maximum value of K1; occurs at the minimum power coefficient, and, since the power coefficient varies inversely as 'N D the maximum value of & ND

will be obtained when the value of ND is a minimum and the propeller diameter is a maximum. Propeller static thrust is conventionally used as a measure of the power plant take-off performance, i. e., the higher the static thrust the greater the acceleration during the take-off run. In the particular airplane illustrated, the conventional direct-drive propeller diameter would be of the order of 6 feet and the engine maximum R. P. M. would be around 2800, whereas in accordance with the invention the airplane illustrated has a geared-down 10 ft. propeller operating at 1000 R. P. M. The static thrust for the smaller propeller for, say, a 100 H. P. engine, would be 410 pounds, and for the large propeller, 750 pounds,

some 78% greater, giving the same static thrust effect as having 178 H. P. driving the smaller propeller. The increase in thrust is limited only by the maximum propeller diameter that can be swung on the airplane. 1

Particularly in regard to military aircraf though also in civilian aircraft, it is advantageous to use a reduced propeller tip speed, not to exceed 700 ft./sec. and preferably as low as .550 ft./sec. With such tip speeds, the engine noise is predominant and mufllers may be used to good advantage. Thus a slow turning geared-down propeller must be used with the conventional light aircraft engine to achieve quiet operation.

In line with the foregoing, I have found that my novel airplane, satisfying the requirements above set forth, may be secured with presentday normal power loadings for take-off brake horsepower (l0 lbs/H. P. or more, up to 14-18 lbs/H. P.) and a maximum wing lift coeflicient for take-off and climb, as well as for approach and landing, preferably of at least 2.5 power-off, in which airplane Propeller blade area propeller diameter Wing area wing span is greater than 0.006, and preferably 0,0075, or more, and in which the ratio (Propeller diameter) Brake horsepower is greater than 0.5, with propeller tip speeds not greater than 700 ft./sec., and preferably less than 550 ft./sec. An example of the application of the two formulae to the specific airplane herein shown is here set forth, employing in the first formula the same figures for wing area and wing span as those for the standard J-3 Cub .(equipped with the said high-lift devices, 100 H. P. engine,

and geared-down 10 ft. propeller above referred to), the propeller blade area of 4.75 sq. ft. being an average conventional propeller blade area from published well-known standard data, and

also employing the figures of said 10 ft. propeller airplane brake horsepower is always figured as take-01f brake horsepower, i. e. the maximum brake horsepower for which the engine is rated.

Thus, in my single-propeller small-field airplane -with its high-thrust propeller high-lift wing and normal power loading, the high-lift of the Wings ismade available for the necessary short take-off and steep climb as well as for lowspeed :approach and low-speed landing. Naturally, the greatest utility of the invention is secured at thelow end of the speed range and at high angles of attack which contributes to slow flight for both take-off and landing, particularly inthe case of airplanes of low take-off and landing speeds of the order of 25-30 M. P. H. Very considerable advantages, however, may be had in airplanes having somewhat higher take-off and'landing speeds, though the advantages are progressively less as said speeds are increased.

Having described the invention, I ciaim:

1.-A single-propeller airplane of normal power loading for small-field use having, in combination, a body, a fixed wing, a high-lift device mounted on said wing, the wing and said device together providing a maximum lift coeificient for take-01f and climb as well as approach and landing of at leastv 2.5 power-off, an engine, and a propeller connected to said engine, in which said airplane Propellerblade area propeller diameter Wing area wing span is greater than.0.006,

(Propeller diameter) Brake horsepower is greater than 0.5, and propeller tip speed is less than 700 ft./sec.

2. A single-propeller airplane of normal power loading for small-field use having, in combination, a body, a fixed wing, high-lift devices mounted on said wing including leading edge slats and trailing edge flaps, the wing and said device together providing a maximum lift coefficient for take-oil" and climb as well as approach and landing of at least 2.5 power-off, an engine, and a propeller connected to said engine, in which said airplane Propeller blade areaXpropeller diameter Wing. areaXwing span is greater than 0.006,

(Propeller diameter) Brake horsepower is greater than 0.5, and propeller tip speed is less than '700'ft./sec.

3. A single-propeller airplane of normal power loading for small-field use having, in combination, a body, a fixed wing, flaps on the trailing edge of said wing, the wing and said fiapsitogether providing a maximum lift coefficient for take-off and climb as well as approach and landing of at least 2.5 power-off, an engine, and a propeller connected to said engine, in which said airplane Propeller blade area propeller diameter Wing areaXwing span isgreat'e'r than 0.006,

(Propeller diar'n'eter) Brake horsepower is greater than 0.5, and propeller tip speed is p less than 700 aft/sac,

4.'A single-propeller airplane of normal power loading for small-field use having, in combination, a body, a fixed "wing, at high-lift device mounted on said wing, the Wing and said device together providing a maximum lift coefficient for take-off and climb as Well as approach and landing of at least 2.5 power-off, an engine, and a propeller connected to said engine, in which said airplane Propeller blade areaXpropeller diameter Wing area'Xwingspan is greater than 0.006,

(Propeller diameter) Brake horsepower is-greater than 0.5.

5. A single-propeller airplane of normal power loading for small-field use having, in combination, a body, a fixed wing, a high-lift device including trailing edge flaps mounted on said wing,

the said wing and said device together providing with said flaps depressed maximum lift coefficient for take-off and climb as well as approach and landing, an enginegand a geared-down pr0-' peller connected to said engine, in which said airplane Propeller blade area-X propeller diameter Wing areaXwing span is greater than 0.006,

(Propeller diameter) Brake horsepower Propeller blade areaXpropeller diameter WingareaXWing span is greater than 0.006,

(Propeller diameter) Brake horsepower is greater than 0.5.

o'r'ro c. KO'PPEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED' STATES PATENTS Number Name Date 2,024,853 Gaines Dec. 1'7, 1935 2,322,745 Rogallo June 29, 1943 OTHER REFERENCES Aircraft Propeller D:sign (Weick), Mc-

Graw-Hill, 1930, pp. 171', 266-273. 

